The invention relates to printing apparatus and particularly to printing apparatus of the type in which a print row is defined by relative movement between a single impact print head and a platen. That is, printing apparatus in which the print head either moves along the length of a fixed platen supporting a print receiving sheet to be impacted by the head or the platen carrying the paper sheet moves past a stationary print head. The sheet in both cases is incremented from one print row to another by a friction feed mechanism provided either in part by the platen itself or by a separate tractor feed mechanism engaging sprocket holes along the edges of the sheet. It is usual for printers fitted with tractor feed mechanism to be of the type in which the platen is stationary and the print head moves across the print row. A print ribbon positioned between the print head and the platen is supported by a ribbon feed mechanism which increments the ribbon during print operations in the usual manner. In some forms of apparatus the ribbon feed mechanism is mounted on the carrier supporting the print head. The ribbon may either be supplied on a supply spool which is placed on a receiving shaft with the ribbon manually theaded around ribbon guides to a take-up spool or, as is often the case with modern printers, supplied in a disposable cartridge which is merely loaded into a cartridge receptacle included on the print head carrier.
Typical of this type of printer apparatus is the so-called wire matrix printer which is of the moving head type and produces printed characters or images by means of a series of dots formed within a matrix. Many such matrix printers are commercially available, one example of which is the IBM 5103 printer. (IBM is a Registered Trade Mark of International Business Machines Corporation). Many other forms of impact printing apparatus of the traversing head type are to be found described in the literature or commercially available in the market place. Instead of the wire matrix print head other single element print heads such as the so-called "daisy-wheel" and "golf ball" print heads are used.
The growing use of colour displays has highlighted the need for a hard copy output device that can reproduce a multi-coloured screen image. Currently available colour printers utilizing ink jet or xerographic techniques, for example, tend to be expensive when compared with monochrome printers using matrix or other single element print heads. Accordingly, consideration has been given to ways of modifying existing relatively low cost monochrome printers to enable them to print multi-colour images. The limited resolution of present colour Cathode Ray Tube technology is well matched to the capabilities of wire matrix impact printer technology and this type of printer has been found particularly suited for such modification.